In special relativity the discrete transformations of the Dirac field \(\psi \) , written in our phases convention, are the parity, \({\cal P}\psi (t,{\bf x})=\gamma ^0\psi (t,-{\bf x})\) , charge conjugation \({\cal C}\psi (t,{\bf x})=i\gamma ^2\psi ^*(t,{\bf x})\) , and Wigner's time reversal, \({\cal T}\psi (t,{\bf x})=-\gamma ^1\gamma ^3\psi ^*(-t,{\bf x})\)  [1]} that closes the famous PCT theorem, \({\cal PCT}\psi =\gamma ^5 \psi (-t,-{\bf x})\)  [2]}. As \({\cal T}\)  is not related to an isometry, one uses often the transformation \({\cal CT}\psi (t,{\bf x})=\gamma ^0\gamma ^5\psi (-t,{\bf x})\)  as an alternative time reversal [3]}. The transformations \({\cal P}\) , \({\cal CT}\)  and \({\cal PCT}\)  are produced by the global discrete \(O(1,3)\)  isometries \(P\) , \(T\)  and \(PT\)  transforming the Cartesian coordinates \((t,{\bf x})\)  into \((t,-{\bf x})\) , \((-t,{\bf x})\)  and respectively \((-t,-{\bf x})\) .
